1. Field of the Invention
The present invention relates to an ink jet recording head for recording by discharging ink to a recording medium. The invention also relates to an ink discharge method.
2. Related Background Art
In recent years, it has become easier for an ink jet recording apparatus to obtain high-quality characters and images. The ink jet recording apparatus has been widely used as the output equipment for a computer in particular. Among such apparatuses, the one that adopts the bubble jet method, in which ink is discharged from the nozzle by means of abrupt changes of pressure exerted with quick boiling given to ink in the nozzle, makes it easier to arrange a number of nozzles in high density with a simple structure, hence becoming the main stream of the ink jet recording apparatuses.
Further, in recent years, along with the wider use of the ink jet recording apparatus, there have been more demands in a higher performance thereof, particularly in terms of the quality of recorded images, and higher speed of recording as well. In order to enhance the image quality, it is important to make the diameter of each dot to be recording on a recording medium (a recording paper sheet, in particular). The demand in such aspect is greater for the recording of images represented by photographs than the recording of written documents. For example, when a written document is recorded, the resolution needed for a beautifulness of a character or for a small character is 600 dpi to 1200 dpi, and the liquid droplet to be discharged should be good enough if it has a dot diameter of 80 to 90 μm (in terms of volume, it is approximately 30 pl). Against this, in the case of recording images, there is a need for providing a resolution of 1200 dpi to 2400 dpi in order to enable the gradation to be as smoothly represented as comparable to a silver salt photograph. When recording is made in such a resolution, it is required to use two kinds of ink having a difference of approximately ¼ to 1/6  in the densities of dye-stuffs used depending on the densities of images if the dot diameter of liquid droplet to be discharged is 40 μm (in terms of volume, it is approximately 4 pl). If the dot diameter of liquid droplet to be discharged is made smaller to 20 μm (in terms of volume, it is approximately 0.5 pl), the density in the high-density portion and the smoothness in the low-density portion are made compatible with one kind of ink having a single density. As described above, in order to obtain the image quality comparable to that of a silver salt photograph, it is a prerequisite that the liquid droplet to be discharged is made smaller.
As the known methods for discharging small ink droplets stably, it has been disclosed, respectively, in the specifications of Japanese Patent Application Laid-Open No. 04-10940, Japanese Patent Application Laid-Open No. 04-10941, Japanese Patent Application Laid-Open No. 04-10942, Japanese Patent Application Laid-Open No. 04-12859, and Japanese Patent Application Laid-Open No. 11-18870 that the heat generating member is arranged close to the discharge port to bring ink to the boil, and bubble thus generated communicates with the outside to minimize the instability of the volume of liquid droplet by means of negative pressure exerted at the time of bubble shrinkage, and then, high discharge energy is provided for the liquid droplet. Such known methods are excellent in discharging small liquid droplets stably. Here, however, liquid droplet is formed by allowing bubble to be communicated with the outside. Then, the shape of liquid surface may affect the shape of liquid droplet, discharge speed, and discharge direction when the bubble is communicated with the outside.
For example, if the bubble is communicated with the outside at the time of bubble growth as disclosed in the specification of Japanese Patent Application Laid-Open No. 11-188870, the liquid column that follows the discharged liquid droplet tends to be in a state of being connected with the side wall of the discharge port on one side. As a result, the cut-off and separation of the main liquid droplet is executed in a state of being displaced from the center of the discharge port, and errors occur in the direction of discharge. As a method for preventing this occurrence, it is disclosed in the aforesaid specification of the Japanese Patent Application that the bubble is communicated with the outside (atmospheric air) when it is shrunk, and then, the separation of the main liquid droplet is executed on the side near the heat generating member so as not to allow the liquid column to be connected with the side wall for the discharge of the main liquid droplet from the center of the discharge port. In this way, it is attempted the enhancement of the directional precision of discharge according to the disclosure therein.
However, in order to attempt making liquid droplet smaller still and enhancing the recording resolution, there is a need for further improvement of the precision in the direction in which liquid droplets are discharged. Also, as another problem here, there is such a case that as shown in FIGS. 9A and 9B, the center of the heat generating member 1102 arranged on the substrate 1101 and the center of the discharge port 1104 formed for the flow path formation member 1103 are displaced eventually due to the variations created in the manufacturing process, because the structure of the ink jet recording head is so minute.
The bubble has a character that it becomes a hemisphere having high central portion by the surface tension as it is grown from the flat shape immediately after boiling. As a result, the liquid surface of the bubble on the central portion thereof is closest to the interface with the outside, and the communication with the outside tends to occur easily on this portion. The central portion of bubble is identical with the center of the heat generating member 1102. Therefore, if the relative positions of the heat generating member 1102 and the discharge port 104 are displaced, the communicating position for the bubble and the outside is biased to allow the tailing end of the liquid droplet to be in a state of adhering to the wall surface of the discharge port 1104. The micro liquid droplet formed on this tailing end portion is allowed to fly at slow speed in the direction different from that of the main liquid droplet due to the viscous resistance thereof to the wall surface of the discharge port 1104. Then, as shown in FIGS. 10A and 10B, this droplet is placed at a position away from that of the main liquid droplet on a recording medium to spoil the image quality. Particularly, the liquid droplet, which is smaller than the conventional one, is easier to be affected by the viscous resistance. Moreover, the discharge direction thus displaced may exert greater influence on the image to be formed. Therefore, it is required more than ever to make arrangement so that such displacement is not easily made in the discharge direction. Here, a reference numeral 1105 designates an ink flow path, and 1106, an ink supply path.
Also, for the ink jet recording head that forms small droplets to be discharged, it is necessary to increase the frequency of liquid droplet discharges per time. As a result, the amount of electric current that runs on the heat generating member is significantly increased, and the voltage drop is intensive due to parasite resistance on the wiring portion up to the heat generating member, leading to a problem that the discharge efficiency is lowered. In order to prevent this, it is effective to adopt a method for reducing the value of electric current by increasing the resistive value of the heat generating member. For the attainment of such means, it is conceivable to increase the resistive value of the material used for the heat generating member. However, there is a limit to the increased value of resistance that may be attained by changing materials of the heat generating member. Also, when a new material is used, it is necessary to obtain a sufficient verification to ascertain whether or not there is any functional problems when it is adopted, thus making it difficult to implement this preventive means.